Using the extension is as simple as opening a new message window and selecting your SMTP server from the list, or adding a new one on the fly with the handy New SMTP Server item, which saves to your account settings for next time—a feature the more well known SmtpSelect extension doesn't have. This extension is useful for anyone that moves their laptop around, as SMTP servers are often blocked by internet service providers to combat spam, and taking a trip to account settings each time is a serious annoyance.

SendVia is a free experimental add-on (meaning you will need a username and password to download it from Mozilla Add-ons). For more useful tweaks for your email experience, see Gina's list of 8 killer extensions for Thunderbird.